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Reactions Involving Free Radicals
Free radical reactions involve one electron species,
frequently generated by homolysis (shown below).
[Note the use of an arrow with a half head to designate the movement of one
electron.]
Weak bonds, like O-O bonds and X-X bonds undergo such
homolysis reactions relatively easily.
Hydroxy radicals (HO.) and hydrogen radicals (H.) are relatively unstable.
However, radicals can be stabilized by resonance, and shielded from reaction by
steric hindrance, sometimes resulting in long-lived radicals.
BHT, above, is sometimes added as a food preservative, to prevent the process of
auto oxidation, that occurs in the presence of air and light. Once it reacts with a
peroxy radical (above) it transfers a hydrogen atom to produce a resonancestabilized, hindered oxygen radical, which does not react further.
Another Stable Free Radical: TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl)
TEMPO is available commercially, but expensive
TEMPO is often used catalytically, together with stoichiometric
amounts of inexpensive oxidants, to oxidize alcohols to aldehydes
and ketones.
The mechanism of this reaction involves oxidation of the N-O
bond to an N=O bond by the secondary stoichiometric oxidant,
which undergoes addition of the alcohol, as shown below.
TEMPO is also used to initiate controlled radical polymerization
reactions
Geometry of Carbocation, Radical,
and Carbanion
Radical reactions can be divided into three steps:
1) Initiation
2) Propagation
3) Termination
Weaker bonds are more readily cleaved by homolysis
Some bonds can be cleaved by heating
Radical
Initiators
AIBN
Benzoyl Peroxide
Link
One common reaction of radicals is an atom abstraction, with a hydrogen atom being
one of the most commonly abstracted atoms (note this process does NOT involve a
hydrogen radical).
Radical Bromination
Link
Allylic (and benzylic) bromination with NBS
(N-Bromosuccinimide)
NBS is regarded as a source of trace amounts of Br2 via the mechanism shown
below.
Note that the free-radical bromination occurs at the benzylic (and allylic)
positions, through resonance-stabilized radical species.
Reductions utilizing tributyltin hydride as
a hydrogen atom donor
The Sn-H bond is relatively weak (82 kcal/mole), relative to the C-H bond (99
kcal/mole)
By contrast, tin forms stronger bonds to bromine, iodine, and sulfur than does carbon.
C-S
C-Br
C-I
C-H
65 kcal
68 kcal
51 kcal
99 kcal
Sn-S
Sn-Br
Sn-I
Sn-H
111 kcal
132 Kcal
56 kcal
82 kcal
The use of TRIBUTYLtin derivatives has become relatively
common. The three butyl groups add molecular weight to
the tin and thus lower its vapor pressure. (Tin compounds
can be toxic by inhalation.)
While a clean, high-yielding reduction of an alkyl
halide is synthetically useful, it would be even more
useful if there existed a method for reducing an
alcohol to an alkane.
The Barton-McCombie
Deoxygenation
Mechanism
Notice that the tin radical attacks the sulfur to generate the highly stabilized tertiary
radical. (This is unlike the addition of nucleophiles to the C=O, which always
occurred at the carbonyl carbon, due to the polarized nature of the carbonyl group.)
Obviously the hydrogen atom is transferred from tin to carbon
relatively easily. Can other alkyl groups be transferred from tin
to carbon (by a free-radical process)?
Radical Reaction with
Allyltributylstannane
One alkyl group that transfers (from tin to carbon) well is the allyl group, as shown
below.
Mechanism of reaction of allyltributyltin with alkyl
halides
(Notice that another common reaction of free radicals is
addition to a C=C.)
The Barton Decarboxylation
In the Case of a (non-radical) Decarboxylation ,Recall that:
Decarboxylation produce CO2, and are thus thermodynamically favorable.
These reactions proceed most readily when the COOH moiety is attached to an
electronegative atom, like oxygen or nitrogen.
Unstabilized Carbanion is
NOT formed
However, even in the case of simple carboxylic acid, decarboxylation may proceed
readily, if the resultant carbanion is stabilized by resonance, or other electronegative
substituent, as shown for acetoacetic acid below.
Stabilized (enolate) carbanion is
formed
The reaction probably proceeds through a cyclic transition state as shown below.
Carboxyl RADICALS, however, decarboxylate much more readily
than the corresponding acids and carboxylate salts.
An early decarboxylation utilizing this fact was the Hunsdiecker
reaction, shown below:
Mechanism of the Hunsdiecker Reaction:
Derek Barton developed a more reliable procedure for
decarboxylation which does not employ the use of Br2.
Mechanism of the Barton Decarboxylation:
Addition of radicals to double
bonds
Under certain conditions, addition of H-Br (but
not H-Cl or H-I) gives anti-Markovnikov
regiochemistry… Why?
Benzoyl Peroxide
Ascaridole
Morris S. Kharasch
LINK
Mechanistic Reason for Effect of Peroxides on the
Regiochemistry of Addition of H-Br to the alkene
Free Radical Polymerization
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